The present invention concerns the field of electromagnetic based detection and sensing, in particular in an industrial environment, and relates particularly to an inductive proximity sensor or switch.
More specifically, the present invention concerns an inductive sensor of the type comprising:                an inductive coil defining a front working plane of the sensor and associated with a covering plate or a plane part of a housing, said plate or part being made of a non magnetic metal with low electrical conductivity, said plate or part being disposed perpendicularly to the coil axis and parallel to its front working plane,        means for supplying the coil or inductance repeatedly with current,        means for processing signals which correspond to the voltages induced in said coil or inductance when fed, said induced voltages being influenced by the presence of objects or bodies within a given detection area, depending on their distance and on their constituting material(s), and said coil or inductance being part of a LC circuit.        
Inductive proximity switches using a resonant circuit are already known. The working principals of this type of sensors are as follows.
When no metallic object is near the sensor (in the detection area of the sensor), the measuring circuit comprised of an oscillator working at resonance conditions oscillates at its maximum amplitude. When a metallic object approaches the active part of the switch, it generates losses by Foucault currents induced in said object and thus a decrease of the amplitude of the resonance.
By comparing this decreased amplitude with a reference value it is possible to detect the presence of metallic objects.
An example of a sensor of the type described before is disclosed in U.S. Pat. No. 4,893,076.
The main disadvantage of the sensor proposed in said U.S. document resides in the necessity of an oscillator of high quality, to provide a sensor launching with a relatively low threshold. This solution can not be used with detectors in fully metal housings, as in these cases the quality of the oscillator would strongly decrease (because of the additional energy absorption in the metallic face part).
This phenomenon is illustrated by comparing FIGS. 7A and 7B of the enclosed drawings, which show the oscillograms of the voltage from two identical LC circuits with identical coils. The only difference between the devices which provided the signals shown in these figures is the presence of an additional stainless steel non magnetic plate (1 mm thick) in the device providing the signal of FIG. 7B, said plate having been placed on the coil of the LC circuit of said device.
Thus, one can clearly recognise that the stainless steel plate modifies not only the quality of the oscillator but also the frequency of the signal oscillations. This metal part must therefore be treated not only as a mechanical or physical part of the sensor but also as an essential part of the electric components forming the oscillating circuit, said plate strongly influencing on the quality and the frequency of the oscillator.
Another type of inductive proximity switch is known from European patent specification EP-A-0 492 029. In this proximity switch, a coil is directly supplied with periodical transmitting current pluses. The effective signal is obtained by a suitable electronic circuit which responds to the received voltage induced in said coil by the decaying of the current which previously flows in the detected body due to the voltage induced therein by the transmitting current pulse.
In an embodiment of the proximity switch known from EP-A-0 492 029, the aforementioned voltage induced in the coil is integrated over a relatively short time window in order to obtain a smoothened useful signal after suitable processing. This signal is compared to a reference voltage in order to generate an output signal which signals the presence of a detected body in the vicinity of the proximity switch if such a body is present. Such a proximity switch provides a good switch distance in the detection of metallic bodies, e.g. of aluminium or steel.
Nevertheless, the inductive proximity switch of EP-A-0 492 029 needs a precise signal generator able to deliver the periodic current to the coil and evaluates measurement values acquired almost immediately after interruption of the current in the coil, and thus during a transitional state which is not necessarily reproduced identically from one period to the next. This results in errors in detection and sensing.
Furthermore, U.S. Pat. No. 6,133,654 discloses an inductive proximity switch of the kind described in European patent specification EP-A-0 492 029 which is suitable for use under conditions of mechanical stresses exerted on the active surface of the proximity switch, in aggressive environments, and under high ambient pressure, and which still provides a good switching distance in the detection of metallic bodies, e.g. of aluminium or steel.
In order to achieve the set goal, this US document provides an inductive proximity switch of the kind described in European patent specification EP-A-0 492 029 wherein said coil, said circuit for supplying the coil with a periodical transmitting current, and said means for processing said signals are arranged in a cylindrical housing which is closed on the side of the active surface of the proximity switch and which consists of a metal which is not ferromagnetic and whose specific electric resistance is relatively high.
It is an aim of the present invention to provide an inductive proximity sensor of the type set forth herein before, which overcomes the limitations of the state of the art known from U.S. Pat. No. 4,893,076, EP-A-0 492 029 and U.S. Pat. No. 6,133,654, i.e. which provides accurate and reliable results and is of simple construction.